In the evolution to IMS networks, CS networks may coexist with IMS networks for a period of time. To decrease the operation and management cost, operators may want to deploy services in the IMS domain in a centralized way. In this way, users can access these services no matter whether they access through the CS domain or other IP-connectivity access network (IP-CAN) domains. In the conventional art, however, a media gateway control function (MGCF) can provide only basic session control by interworking with the IMS due to the capabilities of the existing integrated service user part (ISUP) or the bearer independent call control (BICC). In this mode, the IMS domain cannot provide session control and service continuity control for supplementary services.
FIG. 1 shows a structure of a system for accessing a service to an IMS network through the CS domain in the conventional art. As shown in FIG. 1, an access adaptation unit is installed between a visited mobile switching center (VMSC) and a user equipment (UE). The access adaptation unit intercepts a part of session control signaling between the UE or a radio access network (RAN) and the VMSC, converts the intercepted session control signaling into the Session Initiation Protocol (SIP) signaling, and then forwards the SIP signaling to the IMS domain. The access adaptation unit may also convert the SIP signaling from the IMS domain into circuit signaling and send the circuit signaling to the UE through an access network so that services are uniformly controlled in the IMS domain. The access adaptation unit may establish a session bearer via the interworking between the VMSC, the MGCF, and the IMS, or directly control a medium gateway (MGW) to establish the bearer with an opposite device. As shown in FIG. 1, the section between the UE and the access adaptation unit in an entire session is called a CS session leg and the corresponding session control information is transmitted as CS signaling. The section between the access adaptation unit and the IMS network is called an IMS session leg and the corresponding session control information is transmitted as SIP signaling. To perform correct signaling adaptation and conversion between the two session legs, the access adaptation unit needs to keep the correspondence of certain session information during session setup. For example, the session identifier of the CS session leg is transaction identifier (TI) in layer 3 signaling in the CS domain and the session identifier of the IMS session leg is CALL-ID and remote contact address in SIP signaling. The foregoing information may be referred to as conversion-related information.
In the original CS domain, the signaling flowchart after an inter-MSC handover is as shown in FIG. 2. The UE sets up a session with the remote UE through the VMSC-A. The VMSC-A coordinates and controls all the sessions of the UE. The UE transmits session control signaling messages to other remote devices and networks through the path represented by the solid line in FIG. 2. When the UE moves and is handed over to the VMSC-B, the session control signaling path changes and the UE transmits session control signaling messages to the remote network through the path represented by the broken line in FIG. 2. In this case, the VMSC-A still coordinates and controls all the sessions of the UE but the VMSC-B only transparently transmits the session control signaling between the UE and the VMSC-A through the E-interface.
During the implementation of the present invention, the inventor discovers at least the following problems in the conventional art:
The existing solution supports only the functions of accessing UEs to the IMS in the same MSC and exercising service control in the IMS domain. When the UEs are handed over to another MSC, the session control is not supported and no services may be provided for the users in the IMS domain.